I have two route Suggestions I'd like to suggest as they are both a bit different than what we have now being single track routes with passing loops. one is a combined Tarka/Avocet line as I know a few services used to run through from Barnstaple to Exmouth, preferably using FGW 143s and 150s, particularly as this route has some token operated single track bits it would also give the opportunity at a later date to link with a possible steam run Dartmoor railway. The other suggestion would be Exeter St David's to Sailsbury with SWT 158s/159s although it may be a touch on the long side, however being mostly single track and rural it may reduce layers needed and allow for a longer route mostly for similar reasons to above being single track with passing loops
In the US, Tehachapi Pass has a lot of single track runs with passing sidings, especially the one at Walong (Tehachapi Loop). I would love an SP 1950s-60s era Tehachapi with SD7/9s, FP7s, GP7/9s, GP20s, Alco RSD15s, or FM Trainmasters. Or we could get the route set in the 40s with SP Cab Forwards or in the 1970s-early 90s with SD45s, SD45T-2s or SD40T-2s, U-Boats, and GP60s.
Exeter to Salisbury with the caveat it must be in the loco hauled period, preferably the transition between Class 33 and Class 50 haulage. Would love to thrash 50050 up Honiton Bank.